A typical offshore drilling rig has a platform supported on legs and a cantilever mounted on the platform. Mounted on a drill operation end of the cantilever is a drill floor. The cantilever is moveable in a longitudinal direction of the cantilever such that the drill operation end of the cantilever extends beyond the platform, whilst the drill floor is moveable on the cantilever in a direction transverse to the longitudinal direction. As a result, the drill point can only be located within a rectangular area described by the longitudinal and transverse axes of the cantilever and the drill floor.
The transverse distance covered by the drill floor is limited by the width of the cantilever, which is determined by the distance between cantilever beams extending in the longitudinal direction. To obtain a reasonably wide drilling pattern, the cantilever must also be reasonably wide. Another limitation of the conventional cantilever is that the drill floor creates an asymmetric load on the longitudinal cantilever beams, for example, when the drill floor is moved to a maximal distance in the transverse direction and the longitudinal cantilever beam on the side to which this movement takes place is subjected to a heavier load than the other longitudinal cantilever beam. As a result, each longitudinal cantilever beam must be sturdy and heavily constructed.
Attempts have been made to overcome some of these limitations. For example, U.S. Pat. No. 6,171,027 assigned to Marine Structure Consultants discloses a drill floor that is fixedly mounted on the cantilever so that the load on the longitudinal cantilever beams is centrally or symmetrically located whilst the beams for transverse motion are located on the platform. This design has the advantage that the longitudinal cantilever beams can be of a lighter construction and the range of transverse motion is increased. However, the drilling pattern is still limited by the longitudinal and transverse motions of the cantilever.
U.S. Pat. No. 6,729,804 assigned to Itrec B.V. describes a drilling rig having a platform, a cantilever and a supporting cart disposed between the cantilever and the platform. The supporting cart slides transversely on the platform whilst the cantilever slides longitudinally on the supporting cart on four friction reducing bearings. However, the drilling pattern is still limited by the longitudinal and transverse motions of the cantilever.
U.S. Pat. No. 7,083,004 also assigned to Itrec B.V. describes a drilling rig having a platform, a cantilever and a supporting cart disposed between the cantilever and the platform. The supporting cart pivots about a pivot point on the platform whilst the cantilever slides longitudinally on the supporting cart. The drilling pattern is now part sectorial over the edge of the platform as constrained between two adjacent legs of the drilling rig. U.S. Patent '004 also describes a cantilever with the pivot point being moveable in a direction transverse to the longitudinal direction to create an ellipsoidal drilling pattern. FIG. 1 shows the cantilever described in U.S. Patent '004.
WO 2007/043856 filed by Itrec B.V. describes a rig having a platform and a cantilever. The cantilever has longitudinal rails for extending the cantilever beyond the platform. WO 2007/043856 further discloses rail extensions at the inner end of the longitudinal rails. The rail extensions allow the longitudinal rails to be further extended so that the drilling area is increased.
It can thus be seen that there exists a need for another cantilever skidding system for an offshore platform that allows the drill floor mounted on the cantilever to be moved over a large area and yet overcoming the limitations of the prior art systems, such as, the inconvenience in moving the curved rail 444 of U.S. Pat. No. 7,083,004.